This application claims priority to European Patent Application No. 98117368.5 filed Sep. 14, 1998, and Swiss Patent Application No. 1998 2172/98 filed Oct. 27, 1998.
The present invention relates in general to the detection of airborne particles such as smoke. More particularly, the present invention is related to an optical smoke detector operating in accordance with the xe2x80x9cextinction principlexe2x80x9d and a method of compensating for the temperature drift of the detector.
In a typical extinction measuring method as known in the art, a light beam is transmitted along a measurement section which is accessible to ambient air potentially including smoke, and a corresponding sensor signal is compared with a value which corresponds to the absence of smoke. As both scattering and absorption of light by smoke particles contribute to light attenuation or extinction, and as light is scattered by bright particles and absorbed by dark particles, the extinction measuring method has relatively uniform sensitivity to different types of smoke particles and is equally suitable for the detection of smouldering or low-temperature fires (bright particles) and open fires (dark particles).
When the extinction measuring method is employed in spot detectors, i.e., smoke detectors arranged in a single detector housing, the measurement section is much shorter, and thus a greater sensitivity to smoke particles is required of the transmission measurement. For example, for a 10-cm measurement section, an alarm threshold of 4%/m corresponds to transmission of 99.6% as compared with a reference transmission. If transmission values below the alarm threshold are to be triggered, values of, for example, 99.96% transmission must be detectable, which requires a very high degree of stability of the electronic, optoelectronic and mechanical components of the detector. Transmitted light or spot extinction detectors of this type are described, for example, in European Patent Nos. EP-A-0 578 189 and in EP-A 0 740 146.
A principal source for the instability of conventional spot extinction detectors is the temperature dependence of the associated optical bridge and other optical elements. This temperature dependence results from the fact that the optical elements, including the light source, receivers and associated lenses and mirrors, are typically made of temperature-sensitive materials. Conventional optical devices, such as described in European Patent Nos. EP-A-0 578 189 and EP-A-0 740 146, include waveguides, lenses and/or parabolic mirrors that are made of injection molded plastic material subject to deformation at high temperatures. The parabolic mirrors described in EP-A-0 740 146, for example, are made of plastic material that does not expand isotropically with temperature and thus the stability of the optical device is impacted. The conventional lenses and waveguides described in EP-A-0 578 189 are also influenced by temperature and are therefore also unstable.
The above-described limitations and inadequacies of conventional spot extinction and transmitted light detectors are substantially overcome by the present invention, in which a primary object is to provide a device for detecting airborne particles, such as smoke or other aerosols, that is more stable and is less sensitive to temperature dependencies of a corresponding optical bridge. This object is realized by the present invention in that the detecting device includes an optical bridge, a light source, a measurement receiver and a reference receiver as the only optical elements of the device, wherein the optical bridge includes two circular apertures arranged in front of the light source. As a result of the omission of the lenses, wave guides and parabolic mirrors of conventional spot extinction detectors, improved detector stability is achieved along with an appreciable reduction in cost.
In a first preferred embodiment of the detector according to the present invention, the light source of the detector is arranged in a chamber having an air reservoir. The surface area of the chamber is preferably substantially larger than the surface area of the light source. This embodiment offers the advantage that, as a result of the large surface area of the chamber, airborne particles such as smoke particles are slowly diffused into the chamber and are deposited on the chamber wall and not only on the light source.
In a second preferred embodiment of the detector according to the present invention, the measurement path includes at least one partition having an aperture that blocks laterally penetrating, interfering light but does not affect the radiation of the light source.
In a third preferred embodiment of the detector according to the present invention, the optical bridge includes two end sections and a center partition connecting the end sections, the measurement path being formed on one side of the center partition and the reference path on the other side, and the chamber with the light source is provided in one end section and the chambers with the measurement receiver and the reference receiver respectively are provided in the other end section. This embodiment offers the advantage that the optical bridge is integrally manufactured and can be practically integrated in any detector housing.
In a fourth preferred embodiment of the detector according to the present invention, the section of the optical bridge containing the reference path is secured to a plate, preferably to the circuit board supporting the evaluating circuit, and is laterally sealed by two side walls connecting the end sections and the center partition.
In another aspect of the present invention, a method is provided of compensating for temperature drift in a detector device, such as a smoke detector, having a temperature sensitive optical bridge. The method according to the invention includes the steps of heating the light source at different temperatures, storing the output of an optical measurement receiver at the different temperatures to characterize the temperature drift of the optical bridge, and adjusting the measurement receiver output to compensate for the temperature drift of the optical bridge.
Preferably, if the detector includes a light source, a light-emitting diode and a micro-heater attached thereto within a light source housing, then the micro-heater is periodically activated in situ in the assembled or installed detector and in this manner the actual temperature drift curve is measured. If the optical bridge is mounted on a support made of a material having good thermal conductivity and such support is provided with a heater, then the heater is activated within the framework of the manufacturing process of the detector or during a detector inspection and the temperature drift curve is thereby measured.
Another possibility of measuring the temperature drift curve in accordance with a preferred method of the present invention includes the steps of placing the detector in an oven at the end of the manufacturing process, connecting the detector to a data bus, heating the oven and thereby measuring the temperature drift curve.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.